1. Field of the Invention
The present invention relates to an evaporation mask, and, more particularly, to an evaporation mask in which the pitch between adjacent apertures can be kept even when a tension is applied, a method of fabricating an organic electroluminescent (EL) device using the evaporation mask, and an organic EL device manufactured using the above method.
2. Description of the Related Art
EL devices, which are spontaneous light-emitting display devices, provide a wide viewing angle, good contrast, and a high response speed. Accordingly, much attention is being focused on EL devices because they can be used as a next-generation display device.
EL devices are classified as inorganic EL devices or organic EL devices, depending on what material is used to form a light-emitting layer. Organic EL devices have a higher brightness and faster responsivity than inorganic EL devices, and can provide color display, so they are presently being more actively developed.
Organic EL devices include first electrodes formed in a predetermined pattern on a transparent insulating substrate, an organic film formed on the first electrodes by vacuum evaporation, and second electrodes formed on the organic film such that the first and second electrodes cross each other.
In the manufacture of the organic EL devices having such a structure, the first electrodes are typically formed by patterning indium tin oxide (ITO) using a photolithographic method.
Such a photolithographic method can be used before an organic film is formed, but causes a problem when it is used after an organic film is formed. Because the organic film is very sensitive to water, it must be thoroughly isolated from water both while being fabricated and after the fabrication. Consequently, a photolithographic method including an exposure to water during peeling-off and etching of a resist is not suitable for patterning the organic film and the second electrode layer.
This problem is usually solved by vacuum-depositing an organic light emissive material for the organic film and a material for the second electrode layer using a patterned mask. In particular, the second electrode layer can be patterned using a cathode separator, but it is known that a vacuum evaporation method using an evaporation mask is the most appropriate way to pattern a low molecular organic film.
A technique of patterning an organic film or a second electrode layer using a mask is very important in the manufacture of full-color organic EL devices.
Examples of conventional full-color organic EL device coloring methods include a three-color independent evaporation method, in which red (R), green (G), and blue (B) color pixels are independently deposited on a substrate, a color conversion method (CCM), in which a color conversion layer is formed on a light emissive surface using a blue light source, and a color filtering method, which uses a white light source and a color filter. The three-color independent evaporation method has attracted much attention because it is simple to perform, and provides a high color purity and efficiency.
In the three-color independent evaporation method, the R, G, and B color pixels are independently deposited on a substrate using an evaporation mask. Here, the evaporation mask must be made of a material with a low thermal expansion coefficient in order to prevent thermal deformation, and also must be magnetic if the evaporation mask is to be adhered to the substrate using a magnet. More importantly, the evaporation mask must be highly accurate. In particular, the positions of deposited pixels, that is, the widths of pattern apertures, must be highly accurate, and a high accuracy of a total mask pitch is also required. For example, if an organic EL device must have a high fineness of 130 ppi or greater and an aperture efficiency of 50% or greater, deviation in the widths of apertures of the evaporation mask must not exceed ±5 μm, and deviation in the total mask pitch must not exceed ±10 μm.
As shown in FIG. 1, an evaporation mask 10 used to deposit an organic film or electrodes when fabricating an organic EL device is typically supported by a frame 20, so that the evaporation mask 10 is drawn taut. In the mask 10, a plurality of mask units 12, allowing a single organic EL device to be formed thereon, are formed on a single metal thin plate 11.
Because the evaporation mask 10 is thinly formed and minutely patterned, if it is used without any treatment, some parts of it may droop, preventing accurate patterning. Accordingly, as shown in FIG. 1, optimal tension in the x- and y-axis directions is applied to the evaporation mask 10 to obtain a predetermined accuracy of a total pitch (Pt), and the taut evaporation mask 10 is coupled to the mask frame 20. Upon coupling, it is important to not change the Pt accuracy. The coupling of the evaporation mask 10 to the mask frame 20 can be achieved by various methods such as using an adhesive, laser welding, or resistance welding.
Each of the mask units 12 includes a pattern of apertures. As shown in FIG. 1, each of the mask units 12 may have strip-like apertures elongated in the y-axis direction. However, the tension makes it difficult for the outermost apertures of each of the mask units 12 to maintain a predetermined level of accuracy.
FIG. 2 is a cross-section of a mask unit 12 taken along line I-I of FIG. 1, which shows apertures 13 formed in the mask unit 12. As shown in FIG. 2, a shielding portion 14 is provided between adjacent apertures 13, and outermost apertures 13a are each defined by a shield portion 14 and a rib 15 between adjacent mask units.
As shown in FIG. 2, if tension is applied to the evaporation mask 10 having the apertures 13 in the x- and y-axis directions as shown in FIG. 1, an edge portion 15a of the rib 15 may curve upward. Such deformation of the edge portion 15a of the rib 15 degrades the accuracy of the widths of each of the outermost apertures 13a. Hence, the accuracy of deposition performed on an organic emissive film through the outermost apertures 13a is degraded, and, consequently, accurate patterning of the organic emissive film is not accomplished outside a panel. If an edge of a rib between adjacent mask units is deformed, the deformed rib contacts the organic emissive film, thus generating a defect such as a dark point or a pixel short-circuit on the perimeter of a panel.
As shown in FIG. 3, this problem affects the outermost mask units more than other mask units, thus degrading the accuracy of the total mask pitch.
In other words, as shown in FIG. 3, particularly, mask units 12a and 12b located at the outermost sides in the direction perpendicular to the direction of the length of apertures 13, that is, in the x-axis direction where tension is applied, may be deformed more seriously than other mask units 12 due to the tension applied in the x-axis direction. Accordingly, the accuracy of a total pitch (Pt), the gap between a line 16a connecting the edges of the outer ribs of the mask units 12a and a line 16b connecting the edges of the outer ribs of the mask units 12b, decreases, resulting in degradation of the accuracy of patterning of the mask units 12.
A mask that overcomes the problem of the deformation of strips that form slits due to thermal expansion of the mask is disclosed in Japanese Patent Publication No. 2001-247961. The disclosed mask includes a mask portion and a screen portion. The mask portion is an evaporation mask used to form a patterning film on a substrate by evaporation, and has partitions for defining a plurality of first apertures. The screen portion contains a magnetic material and has a plurality of second apertures smaller than the first apertures. The second apertures are disposed on the first apertures of the mask portion.
Japanese Patent Publication No. 2001-273979 discloses the structure of a magnetic mask. Japanese Patent Publication No. 2001-254169 discloses an evaporation mask frame assembly in which a patterned mask masks a deposition area while adhering closely to a material to be deposited, and has fine gaps and fine patterns. The fine patterns of the patterned mask are supported by fine ribs.
These disclosed masks adhere closely to a material to be deposited because they are formed of a magnetic material. However, these masks still have a problem of losing accuracy due to the deformation of the outermost apertures during a tensing operation.
Japanese Patent Publication No. 2002-9098 discloses a pattern forming apparatus for preventing a pre-formed film on a substrate from being damaged due to a mask coming partially off of a frame due to a thermal expansion during evaporation. The pattern forming apparatus includes a support which is formed to be larger than the mask, and has a dent portion onto which the mask is seated. The support prevents a mask from becoming rippled due to thermal expansion during the formation of a film. Also, by forming a magnetic element on the side of the mask facing away from the support, the magnetic element makes the mask closer to the substrate so that a space between the mask and the support is created. This space contributes to cooling the mask.
However, because the disclosed mask having slits is not firmly supported by the frame, the location of the mask cannot be accurately controlled. In particular, in the case of organic EL devices that must have very thin evaporation masks to accomplish highly accurate patterning, the location of the evaporation mask may be changed during evaporation.
Japanese Patent Publication No. 2002-8859 discloses a pattern forming apparatus for preventing a mask from being expanded by heat during the formation of a film, in which a liquid path is formed within a frame which supports the mask, and a cooling solution circulates within the liquid path. However, this invention also overlooks the possibility of tensions and apertures becoming inaccurate during fixation of the mask into the frame.
Japanese Patent Publication Nos. 2000-48954, 2000-173769, 2001-203079, and 2001-110567 disclose metal masks including supplementary lines to prevent drooping of a mask shield between the mask and a frame. Like the above-disclosed masks, these masks may be stretched and warped while being fixed onto a frame after applying tension in order to accomplish highly accurate patterning.